Fiber-optic telecommunication networks are constantly being upgraded to carry more and more channels over a single optical fiber. And while fiber manufacturers may not applaud this trend because it decreases the demand for optical fiber, it clearly signals the direction of this industry. Indeed, Lucent Technologies Inc. recently announced an optical system having over 100 channels, each transmitting 10 gigabits of data at a different wavelength, over a distance of nearly 250 miles (400 km) using its TrueWave200 fiber. This represented the world's first long-distance, error-free transmission of a terabit (1 trillion bits) of information per second over a single strand of optical fiber. Associated with such multi-channel optical systems are wavelength division multiplexers (WDM), which operate to combine an number of separate and distinct wavelength regions (channels) onto a single optical fiber in one direction of transmission, and to separate them from the optical fiber in the other. This is to say that the WDM operates as multiplexer in one direction of transmission, and as a demultiplexer in the other. These channels each have a different central wavelength (i.e., .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.n) and, for optimum performance of the WDMs and associated transmitters and receivers, it is important that the optical signal power of each channel be precisely controlled, and preferably equal to all others.
Known optical attenuators are generally expensive and have unacceptably large variation between supposedly equal units. Large variations exist when the attenuators leave the manufacturer, and variations even occur during the life of an attenuator that degrade the quality of an optical transmission system. Additionally, many known attenuators are unable to handle high optical power levels without damage.
One of the limitations encounter by optical fiber systems relates to optical noise. A common source of optical noise is optical power reflections. Optical power reflection generally occurs at any discontinuity in a fiber optical transmission path, including the end of an optical fiber, and causes a portion of the incident light to travel back toward the source. Optical power reflected in this manner may reflect again when it arrives at the source point or other points of discontinuity in the system, adding an unwanted noise component to the signal. Optical power that is reflected back into a source can also corrupt the fundamental operation of the source, typically a laser.
Low reflectance attenuators are known in the art and one is described in U.S. Pat. Nos. 5,082,345 and 5,274,729 in which an attenuator disc, made from polymethylmethacrylate (PMMA) plastic, is slidably suspended from a longitudinal slot in an alignment sleeve. Spring-loaded optical plugs are inserted into opposite ends of the sleeve and engage opposite sides of the disc to provide between 5 and 20 dB of attenuation depending on thickness. However, it becomes increasingly heroic to manufacture a disc whose thickness is less than about 0.2 mm, where the insertion loss is about 5 dB.
Another known optical attenuator comprises a length of doped optical fiber that is more lossy than conventional optical fiber. However, the insertion loss of such doped fiber is about 10 dB/cm, and the loss is not uniform over an entire spool. As a result, measurements are tedious and the ability to fabricate attenuators to within .+-.0.5 dB of a desired value is not practical.
Variable Attenuator Connectors (VACs) are known that combine connector and attenuator functions. In such VACs, a finely machined self-locking lead screw mechanism enables linear motion of the connector ferrule, and this creates an air gap (hence attenuation) between plug connectors. In effect, the ferrule within the VAC is being withdrawn from contact with the ferrule of another plug connector. The level of attenuation is set by rotating a nut at the end of the VAC. Back reflection is reduced when the end faces of the mating ferrules are cleaved at an angle. One such device is commercially available from Johanson Manufacturing Corporation. However, the air-gap separation between mating ferrules is subject to variation due to vibration and creep. Even small variations in air-gap separation cause substantial changes in attenuation (e.g., the optical signal power doubles if the air gap decreases by about 50 microns).
Accordingly, what is desired is an optical attenuator that can be inexpensively manufactured with high precision (e.g., within 15% of its nominal value) over a wide range of attenuation levels (e.g., between 0 and more than 20 dB), and whose attenuation level does not vary when subjected to vibrations. Moreover, it is desirable that the optical attenuator be able to withstand high optical power levels (e.g., more than 20 dbm) without damage. Finally, it is desirable to provide an optical attenuator that is suitable for use in a WDM system where precise amounts of insertion loss (attenuation) are needed for optimum performance of its associated multiplexers, demultiplexers, transmitters and receivers.